Add bootstrap responsive embed to wordpress automatic media embed and iframes

functions.php

<?php
// your custom code...

add_filter('embed_oembed_html', 'my_embed_oembed_html', 10, 4);
function my_embed_oembed_html($html, $url, $attr, $post_ID) {
    $iframe = str_replace('<iframe', '<iframe class="embed-responsive-item"', $html);
    $newHtml = '<div class="embed-responsive embed-responsive-16by9">%s</iframe></div>';
    $html = sprintf($newHtml, $iframe);

    return $html;
}